"Double-conversion," "single-conversion," "off-line standby": What do these static UPS operating terms mean? And what about inverter technology terms such as"step wave," "pulse-width modulation," and "ferroresonant?"

Yes, while choosing UPS equipment can be confusing, the wrong equipment choice can result in unexpected downtime and lost profits. With the various technologies and different types of operation available in our industry, you must learn what your choices are to make a knowledgeable decision. Here's some basic information to help you out.

The term "double-conversion" describes a UPS that converts AC to DC and then back to AC. The inverter of this UPS system continuously carries the load; as such, it is sometimes called a true on-line UPS. You can configure all static UPS technologies as double-conversion systems.

Advantages of double-conversion include:

  • Superior voltage and frequency regulation at the UPS output,

  • Isolation of the load from the utility source, and

  • Quarter-cycle transfer capability with the use of a static transfer switch in the event of UPS failure.

Disadvantages, compared to other topologies, include:

  • Lower efficiency and

  • Greater heat dissipation.

The term "single-conversion" refers to the single AC-to-AC conversion normally seen by the load. For example, input to the UPS passes only through an isolation transformer, regulating transformer, or filter. To bring the standby UPS components on-line, an abnormal condition must exist.

There are a variety of single-conversion topologies, including line-interactive and tri-port configurations. In a line-interactive system, the inverter interacts with the line to buck, boost, or replace incoming power on an as-needed basis. The inverter may either operate continuously or switch on by control logic.

A tri-port system is similar, but is always on line. The utility power normally passes through an isolation transformer and filter or a regulating (ferroresonant) transformer.

Advantages of a single-conversion system include:

  • Increased efficiency and

  • Lower heat loss.

Disadvantages include:

  • Inferior voltage and frequency regulation, and

  • Possible complete interruption of power to the load upon loss of normal AC input.

An off-line UPS operates only when it detects loss of the normal utility source to the load. Advantages of the off-line UPS are:

  • Increased efficiency and

  • Lower heat loss.

Disadvantages include:

  • Certain interruption of power to the load due to the loss of the normal source and

  • The possibility of standby equipment failing to operate properly.

Various inverter technologies are available, including:

  • Step wave,

  • Pulse-width modulation (PWM), and

  • Ferroresonant.

These technologies are the result of different SCR circuits, gate controls, and output filtering techniques.

The power switching stage of each type uses either silicon-controlled rectifiers (SCRs) or bipolar transistors to switch the DC power on and off to replicate a sine wave. When it receives a gate signal, an SCR switches on. The simplified SCR circuit, as shown in Fig. 2, produces a square wave output. The wave's amplitude is twice that of the DC input voltage. Here's how the switching works. First, SCRs 1 and 4 gate (start/close the circuit) simultaneously. Then, SCRs 1 and 4 switch off, and SCRs 2 and 3 gate.

Step wave inverter. Power semiconductors and phase shifting networks create a six- or 12-step "staircase" waveform. You filter this into a sinusoidal shape. It takes a semiconductor device plus a phase shift for each step in the output voltage waveform.

Pulse-width modulation inverter. This type converts DC to AC by using power switching at a 20 kHz to 50 kHz rate. A linear feedback loop is part of the circuitry. The output is a pulse-width modulated positive and negative square wave. A simple output low-pass filter removes the high frequency carrier for a smoothed sine wave.

Ferroresonant converter. This type converts DC to AC by generating a square wave filtered by a ferroresonant transformer, creating a true sine wave output. The ferroresonant transformer (also called an "electric flywheel" because it rides through a one-cycle loss of input) is a "tuned" nonlinear transformer. Magnetic storage of electrical energy in the transformer supplies one cycle of ride-through energy (upon loss of input to the transformer) to allow a zero-break operation of the static transfer switch.

So, which inverter technology is better? Each has its advantages and disadvantages. Let's look at each based on the following criteria.

Response to inverter failure. The step-wave and PWM systems have sensors that see a failure of the inverter at its output. With this, the static switch detects the failure and transfers the load to the bypass source within 1/4 cycle. Nearly all loads are insensitive to this failure detection and transfer duration.

The ferroresonant inverter senses failure in front of and at the output of the ferroresonant transformer. Failure detected at the front initiates transfer of the static bypass switch; completed within a quarter cycle. The transformer has one cycle of stored energy, which provides output ride-through during the quarter cycle failure detection and duration.

At the output of the ferroresonant transformer, a failure at the inverter output results in the transfer of static bypass switch within quarter cycle (similar to the step-wave and PWM inverters).

Overload capability. The output current limit (or overload capability) affects the fault clearing capability of a UPS. Its protective device (fuse or breaker) clears a fault at the load side of the UPS; only if enough fault current passes through the device to cause the fuse to melt or the breaker to trip. Therefore, a UPS with higher overload capability generally has better ability to clear a downstream fault without having to transfer the faulted load to an alternate source with higher available fault current. (The higher current helps assure the protective device operates.)

Step-wave and PWM inverter technologies typically have 150% overload capability, while most ferroresonant UPS deliver 500% of the continuous rated output for one cycle.

Dynamic response. If a UPS is operating near 100% of its rated capacity, and more than 50% of the load suddenly shuts down, the voltage regulation capability of the UPS may suffer. The dynamic response of the ferroresonant UPS is inferior to that of other technologies. But, it's typically within the Information Technology Industry Council's (ITIC's) recommendations. (Note: ITIC was formerly called the Computer Business Equipment Manufacturers Association, or CBEMA.)

Harmonic distortion. There are two harmonic distortions associated with UPS systems. One is the UPS' ability to minimize the amount of harmonic distortion caused by the UPS-fed loads. The other is the amount of harmonic distortion the UPS reflects back onto the distribution source that's providing its input. This second characteristic affects the power quality of other loads.

Input filters can mitigate reflected harmonics at the UPS input. Step-wave and PWM systems have greater sensitivity to load-generated harmonic distortion than other types of UPS systems. Sometimes, you must derate the system capacity to compensate for load profiles with a high crest factor (the ratio between the peak of the current waveform and its rms value).

Ferroresonant technology has superior line regulation (typically 53% regulation with a 15% variation in input voltage), and good power factor correction when connected loads have poor power factor.

Efficiency. Step wave and PWM technologies have greater efficiency than ferroresonant UPS systems, and PWM is more efficient than step-wave technology. Ferroresonant UPS systems have poor efficiency at 50% load.

Environment (Footprint, Weight, Noise, Heat Loss, EMF). PWM systems have the smallest footprint, lightest weight, produce the least audible noise, and generate less heat than other types of UPS technologies. Step wave technology uses more filtering than PWM, and therefore occupies more floor space. Ferroresonant UPS systems are larger and heavier. Earlier ferroresonant UPSs generated considerable audible noise. Although the PWM UPS is quieter, newer ferroresonant units are even quieter than their predecessors.

Due to the high-frequency carrier used in PWM inverter technology, these systems emit electromagnetic interference (EMI). This may disrupt operation of nearby electronics or cause errors in signal transmission of adjacent wiring. Special filtering or shielding can mitigate EMI effects.